Fix Ethernet library link status and missing functionality with recent IDF versions

[ppescher]

Description of Change

The variable eth_link was never updated, so it was always reporting link down.
Proposed fix is to use the status bits managed by WiFi callbacks, that are updated correctly.
Link duplex mode did not have an implementation for IDF versions > 3 and actually the version check was done incorrectly with the preprocessor.
Proposed fix is using the same ioctl mechanism as for the link speed.

Tests scenarios

Tested with PlatformIO and the latest stable release 2.0.5 on a custom hardware using LAN8710 and LAN8740.

Related links

Closes #6105

[me-no-dev]

Thanks @ppescher for the fix!

[TD-er]

Just tested this and it seems to be working fine.
Ethernet connected state is reliable again and also the reported network speed/mode is working fine.

I did test by plugging and re-plugging a number of times and also by selecting the Ethernet link mode on the switch.
Half-duplex, Full duplex and auto is all reported fine. Also 10 vs. 100 Mbps link speed.

However, 10 Mbps half-duplex is not really usable as it seems to loose quite a lot of packets.
But I doubt anyone will ever use that again these days.
When the speed gets limited from the ESP side of the connection, you probably have other issues like unstable voltage to the ESP.

TL;DR
A think this should be merged.